FIG. 1 shows a third generation partnership project (3GPP) long term evolution (LTE) user-plane protocol stack 100. A wireless transmit/receive unit (WTRU) 105 and a Node-B 110 transmit and receive data at different protocols/levels, (e.g., physical (PHY) layer 115, media access control (MAC) layer 120, radio link control (RLC) layer 125, and packet data convergence protocol (PDCP) layer 130). Each protocol layer performs a variety of functions.
The MAC layer 120 provides a hybrid automatic repeat request (HARQ) retransmission functionality whereby a transmitting MAC/HARQ entity retransmits failed MAC/HARQ protocol data units (PDUs), depending on the HARQ positive acknowledgement (ACK)/negative acknowledgement (NACK) feedback that is transmitted by a receiving MAC/HARQ entity.
The RLC layer 125 provides ARQ retransmission functionality whereby the transmitting side of an acknowledged mode (AM) RLC entity retransmits any failed RLC PDUs based an RLC status report transmitted by the receiving side of the AM RLC entity, or based on an indication of a failed MAC/HARQ delivery from a transmitting MAC/HARQ entity.
FIG. 2 is an example illustrating ARQ and HARQ-assisted ARQ operations in LTE. FIG. 2 shows a wireless communication system 200 including a transmitter 205 and a receiver 210. The transmitter 205 includes an RLC/ARQ unit 215 and a MAC/HARQ unit 220. The receiver 210 includes an RLC/ARQ unit 225 and a MAC/HARQ unit 230. The term “transmitter” refers to a transmitting node, which resides in a WTRU for uplink data and a Node-B for downlink data. The term “receiver” refers to a receiving node, which resides in a Node-B for uplink data and a WTRU for downlink data.
An HARQ failure occurs if an HARQ entity does not receive a positive HARQ ACK after a predetermined number of HARQ transmissions. To simplify the example, assume that HARQ delivery failure occurs if the HARQ entity transmits the HARQ PDU twice and does not receive an HARQ ACK.
The RLC/ARQ entity 215 in the transmitter 205 creates an RLC PDU and submits it to the MAC/HARQ unit 220, also in the transmitter 205. The MAC/HARQ unit 220 then transmits a MAC PDU that contains the RLC PDU a predetermined number of times, (e.g. twice), unsuccessfully. Hence, the HARQ process fails to deliver the MAC PDU to the receiver 210. The HARQ process failure triggers a local NACK, (i.e., HARQ assisted ARQ), indication, whereby the MAC/HARQ unit 220 notifies the RLC/ARQ unit 215 of the failed delivery of the RLC PDU. The RLC/ARQ unit 215 initiates an ARQ retransmission of the failed RLC PDU, and submits the retransmitted RLC PDU to the MAC/HARQ unit 220. The MAC/HARQ unit 220 then transmits a MAC PDU that contains the RLC PDU once unsuccessfully. An error may occur on the HARQ feedback, whereby the NACK transmitted by the receiver 210 is erroneously received as an ACK at the transmitter 205. The RLC/ARQ unit 225 in the receiver 210 may transmit an RLC/ARQ status report that positively or negatively acknowledges data, (i.e., ARQ ACK/NACK). The RLC/ARQ status report may be transmitted in several steps, (i.e., via MAC/HARQ, and the like), but for simplifying FIG. 2, it is shown via an end-to-end line. The transmitter 205 checks the RLC/ARQ status report it received, and determines that the RLC PDU is not positively acknowledged. Consequently, the RLC/ARQ unit 215 initiates an ARQ retransmission of the failed RLC PDU, and submits the retransmitted RLC PDU to the MAC/HARQ unit 220. The MAC/HARQ unit 220 transmits the MAC PDU that contains the RLC PDU a predetermined number of attempts, and is successful. The MAC/HARQ unit 230 in the receiver 210 delivers the successfully received packet to the RLC/ARQ unit 225. The RLC/ARQ unit 225 may transmit an RLC/ARQ status report that positively or negatively acknowledges the data. The transmitter 205 checks the RLC/ARQ status report it received, and determines that the RLC PDU is positively acknowledged. Consequently, successful delivery is confirmed, and no further ARQ retransmission is required.
The procedure shown in FIG. 2 is simplified to illustrate an exemplary procedure, however, more functions may be performed. For example, RLC re-segmentation may be performed, whereby instead of re-transmitting a whole RLC PDU in one transmission, the RLC PDU may be re-segmented into multiple RLC PDU segments.
In some implementations, a local NACK (HARQ assisted ARQ) may not be implemented, and in this case an ARQ retransmission will only be triggered via RLC/ARQ status reports.
Recently proposals have been introduced for LTE-advanced, which features additional improvements to LTE. LTE-advanced (LTE-A) will present a significant enhancement over LTE, e.g., peak data rates of 0.5 Gbps in uplink and 1.0 Gbps in downlink.
The use of “relays” is one of the technologies being considered for LTE advanced. FIG. 3 shows exemplary uses of relays in a cellular communication system. When referred to herein, the term ‘relay’ may refer to a “relay node”, or an intermediary node, that may provide a link between a Node-B and a WTRU.
Accordingly, effective, efficient and fast ARQ retransmissions with relays are desired.